The current state of U.S. dependence on foreign oil reserves for much of its energy needs is undesirable in light of the possibility of recurring supply and price disruptions. Accordingly, the Federal Government and the DOE, in particular, have been promoting the development of domestic energy sources, including the abundant supply of inexpensive coal and natural gas. Recent efforts in advanced coal-based technology have focused in part on the use of coal gas as a potentially more efficient and economical alternative to pulverized solid coal. However, the presence of significant quantities of contaminants such as particulates, alkalis, and sulfur compounds presents a serious problem. Sulfur is of particular concern in that it may promote the corrosion of power generation equipment, is a poison to catalysts in fuel cell power plants and gas liquefaction, and is a principal source of air pollution and acid rain. For these same reasons, sulfur is also a problem when present in other gas streams used in energy production such as natural gas.
Since bulk energy production is a highly price driven industry, technologies like polymer membranes and aqueous scrubbers which require cooling of the gasification product streams prior to treatment incur a heavy penalty and are not economically feasible. The current state-of-the-art in high temperature, high pressure desulfurization involves the use of chemical sorbent beds. The materials, generally supported or unsupported metal oxide pallets, remove sulfur from the gas by a chemical reaction of the type: EQU MO.sub.x +H.sub.2 S.fwdarw.MS.sub.x +H.sub.2 O
wherein M is a metal reactive with S. A number of such systems have been studied extensively and demonstrated a high degree of effectiveness. However, as the capacity of the sorbent is approached, its effectiveness drops, and it must be removed from service, making it a batch process with all the associated restrictions and engineering complexities thereof.
Because the metal-sulfur reaction is stoichiometric, large quantities of bulky sorbent must be used. Moreover, although some sorbents may be regenerated by means of highly exothermic reactions using an oxidant, even the regenerable sorbents must be replaced after a number of cycles, thereby introducing replacement costs and the more and more vexing problem of large scale solid chemical waste disposal.
Membrane separation can offer increased flexibility of design, continuous operation, and lower cost, and is rapidly replacing other technologies in many applications. Membranes currently being studied for hot coal gas separations are porous ceramics that can withstand high temperatures and pressures in corrosive chemical environments. See, for example, "Gas Separation Using Inorganic Membranes", Egan et al, Proceedings of the Tenth Annual Gasification and Gas Stream Clean-up System Contractors Review Meeting, Morgantown, W.V. (August, 1990). Since the size of the pores of the ceramic membranes are generally larger than about 40 angstroms, the dominant mechanism of separation must be Knudsen diffusion under the driving force of the differential pressure between the two sides of the membrane. In Knudsen diffusion, hydrogen can permeate through the membrane up to about 4.12 times faster than hydrogen sulfide. Accordingly, the permeate stream is enriched in hydrogen while most of the contaminates stay in the retentate stream. Although the degree of separation may be improved by staging, a significant quantity of fuel will always be lost to the retentate stream. Furthermore, the low pressure hydrogen-enriched permeate must be repressurized after each stage. For these reasons, current passive membrane systems are not presently economically competitive with sorbent-based processes for the removal of H.sub.2 S from gas streams.
It is an object of the present invention to develop a continuous process for the removal of sulfur from gas streams in an economical fashion.